Deadlifts should be one of the more simple exercises to come up with weight for. Since it is not a dynamic move the shape of the weight is less important. As long as you can grip it and lift it while maintaining form you should be good to go.

Duffle bag full of sand or rocks
Small (or large!) child
A log (maybe wrap a couple straps around it to make handles)
Your suitcase
Water jugs
Spare tire off the car

Deadlifts should be one of the more simple exercises to come up with weight for. Since it is not a dynamic move the shape of the weight is less important. As long as you can grip it and lift it while maintaining form you should be good to go.

Duffle bag full of sand or rocks
Small (or large!) child
A log (maybe wrap a couple straps around it to make handles)
Your suitcase
Water jugs
Spare tire off the car

If you lift children, only lift children you do not like, so that if you will drop them it will not be a great loss.

As others have said, there is no real "substitute" for a weighted deadlift, however you can build strong, functionally fit legs with just your bodyweight. For the last few years pistol squats, shrimp squats, back bridges and yes - one legged bodyweight deadlifts - have made up the bulk of my lower body strength training and I've been pleased with the results. The key to maximizing these types of bodyweight movements is to do them slowly and with control through the whole range of motion.

Some people prefer to lift weights and I respect that - weight training is a fantastic way to build strength - but those of us who prefer bodyweight training can still get very strong.

"In theory, theory and practice are the same. In practice, they couldn't be more different."

"You can have anything you want, but you can't have everything you want."

Unstable surface + barbells = pointless idiocy.
Unstable surface + just your bodyweight = whatever floats your boat, sure, why not. But - why, again?
If you don't think that strength gained from a traditional deadlift is useful on, say, a moving boat, then you haven't really weight trained. If training effects were that specific, then barbell programs would never do anyone any good. And they obviously do.
I've never seen a single person go from nasty to buff on a bosu-ball based program.

OP, maybe you can get a truck axle or a transmission at a wrecking yard and start messing with that?